1. Field of the Invention
This invention relates to transmitting a secondary signal with a primary video signal, and more particularly to timing signals to control when the secondary signal is transmitted.
2. Description of the Prior Art
It has been realized for a long time that the video spectrum in common commercial formats, such as NTSC, PAL, and SECAM, is not efficiently used. To overcome this inefficiency, several different video spectrum formats have been proposed. However, these formats, such as Japanese HDTV, cannot be received by the conventional television receivers in common use.
Another approach to efficiently using the spectrum of a video signal, of common formats, is to time multiplex the video signal with a secondary signal. This approach is used for transmitting close captioned information with the video signal. In particular, the close captioned information is transmitted during the vertical blanking intervals. A specially designed receiver may receive and display close captioned information, while commonly available commercial receivers may display the transmitted signal without any perceptible visual defects in the image from the closed captioned information. However, the data transmission rate is low, thus, limiting the amount of information that may be transmitted.
Another method, is to place a secondary signal in the 2-3 Mhz range, which lies between the peak values of the Chrominance and Luminance carriers. An example of this is shown in "A Novel Television Add-On Data Communication System", by Patrick T. King in Vol. 33 of Journal of the SMPTE (January 1974). The method uses odd multiples of one half the scanning rate, which results in the spectral energy of the secondary signal overlapping with the chrominance information. This requires that the transmitted data rate of the secondary signal must remain low (i.e., under 21 kilobits per second) and the secondary signal must be transmitted at low power to avoid interference with the chrominance carrier.
A drawback to all of these approaches is that a secondary signal cannot be transmitted at high data rates without causing interference with the primary video signal. This limits both the amount of data and the type of data which may be sent.
The lower data rates used in these approaches requires a careful balancing of data transmission needs against the need to reduce transmission errors. Any error reduction methods such as error correction coding reduce transmission data rates.
Further, most current transmitting stations switch between different primary video signals, for example between a primary television program and a variety of commercials. This can cause problems in synchronization and can increase transmission errors. Thus, it becomes extremely important to utilize error reduction techniques, even though this will reduce the data transmission capability of the secondary signal transmitter.